1. Field
The subject invention is in the field of static structure which will be moved occasionally, such as the lids of vaults used for underground electrical and sewer systems.
2. Prior Art
Conventional covers used on underground vaults are made of cast reinforced concrete or are made of sheet iron parts welded together or may be some combination of the two. To be strong and stiff enough to tolerate being driven over by all kinds of vehicles, including trucks and heavy mobile machinery these conventional lids are heavy and cumbersome and accordingly may be hazardous to handle, particularly if no machinery is used and even when machinery is used. The hazard and the need for machinery to reduce the hazard both add to the cost of servicing the equipment in the vault covered by the lid. Accordingly there is a long standing need for a vault lid which is considerably lighter in weight than conventional prior art lids and would be available at first and maintenance costs comparable to those for conventional lids. The objectives of the subject invention are to provide such a cover having features which facilitate and reduce the hazards of its use.